A harmonic drive gearing, which serves as a reducer to be used in a conventional drive unit, has a configuration as shown in FIG. 12 (see patent literature 1). That is, in this conventional unit, a flex spline 30 is positioned so as to surround a wave generator 40, which has an outer peripheral surface that is formed to an elliptical shape, across a flexible bearing 50. Also, a circular spline 20, which has a circular inner peripheral surface, is positioned at the outside of the flex spline 30.
Inner teeth are formed on the inner peripheral surface of the circular spline 20, outer teeth, which are engageable with the inner teeth of the circular spline 20, are formed on an outer peripheral surface of the flex spline 30, and the flex spline 30 is enabled to deflect in radial directions. The number of teeth of the flex spline 30 is set to be slightly less than the number of teeth of the circular spline 20.
At its major axis portions, the wave generator 40 deflects the flex spline 30 via the flexible bearing 50 and makes the outer teeth of the flex spline 30 engage with the inner teeth of the circular spline 20.
With a harmonic drive gearing arranged in this manner, when, for example, the wave generator 40 is rotated, the engagement points, at which the outer teeth of the flex spline 30 and the inner teeth of circular spline 20 engage, move in a circumferential direction in accordance with the variation of the positions of the major axis portions of the wave generator 40. In this process, since the number of teeth of the flex spline 30 is slightly less than that of the circular spline 20, when, for example, the wave generator 40 is rotated 360 degrees, the flex spline 30 is rotated in the circumferential direction by the difference in the number of teeth with respect to the circular spline 20.
That is, when the above-described conventional harmonic drive gearing is used as a reducer, in which the wave generator 40 and the flex spline 30 are respectively used as a reducer-input and a reducer-output, the rotation speed of the flex spline 30 is reduced greatly in accordance with the rotation speed of the wave generator 40.
A harmonic drive gearing with such a high speed reduction ratio is used, for example, for generating flexing and extending motions of a leg of a leg type mobile robot. FIG. 13 is a diagram showing a joint of the leg of the leg type mobile robot.
A first link 70 and a second link 90 are joined through a rotary supporting mechanism 91 and are enabled to rotate with respect to each other. A harmonic drive gearing 60 is mounted so that the rotating shaft thereof matches with the center of rotation of first link 70 and second link 90. That is, the circular spline 20 is fixed to the second link 90. The flex spline 30 is fixed, at a first rotating shaft 30a (output shaft), to the first link 70 by means of a plurality of bolts 80.
The wave generator 40 is provided with a second rotating shaft 40a, the second rotating shaft 40a is supported in a manner enabling rotation with respect to the first rotating shaft 30a. A pulley 61, for transmitting a driving force, is fixed to the first rotating shaft 30a at the end section to which the first rotating shaft 30a is not provided.
Here, when the driving force from a motor (not shown) is transmitted to the pulley 61 via a belt 62, the wave generator 40 is driven. Then, the flex spline 30 is rotated at a reduced speed than the rotation speed of the pulley 61 in accordance with the rotation of the wave generator 40, and thus the first link 70 is rotated.
In case of the above-described harmonic drive gearing 60, however, since the position where the flex spline 30 is provided is limited to an end section of the first rotating shaft 30a, the position of the harmonic drive gearing 60 with respect to the first rotating shaft 30a is also restricted to the end section of the first rotating shaft 30a. 
Thus, the harmonic drive gearing cannot be positioned between two sets of bearings, when for example the first rotating shaft 30a is supported by two sets of bearings. This provides the problem of low degree of freedom in terms of layout.
Also, since the pulley 61, which transmits a driving force to the wave generator 40 and is disposed on the second rotating shaft 40a, is positioned next to the wave generator 40, the size in a direction along the rotating shaft inevitably becomes large. This may make worse the usability depending on a usage.
Meanwhile, an apparatus, in which the wave generator is provided at an outer periphery side of the flex spline, has been known (see patent literature 2).
[Patent Literature 1]
Japanese unexamined patent publication H5-141484 (claims, claim 1, and FIG. 1).
[Patent Literature 2]
Japanese unexamined patent publication S59-9336 (FIG. 2).